Method and apparatus for transmitting plcp frame in wireless local area network system

ABSTRACT

A method of transmitting a Physical Layer Convergence Procedure (PLCP) frame in a Very High Throughput (VHT) Wireless Local Area Network (WLAN) system includes generating a MAC Protocol Data Unit (MPDU) to be transmitted to a destination station (STA), generating a PLCP Protocol Data Unit (PPDU) by adding a PLCP header, including an L-SIG field containing control information for a legacy STA and a VHT-SIG field containing control information for a VHT STA, to the MPDU, and transmitting the PPDU to the destination STA. A constellation applied to some of Orthogonal Frequency Division Multiplex (OFDM) symbols of the VHT-SIG field is obtained by rotating a constellation applied to an OFDM symbol of the L-SIG field.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/985,317, filed on Dec. 30, 2015, which is a continuation of U.S.application Ser. No. 14/560,971, filed on Dec. 4, 2014, now U.S. Pat.No. 9,276,791, which is a continuation of U.S. application Ser. No.13/943,572, filed on Jul. 16, 2013, now U.S. Pat. No. 8,937,933, whichis a continuation of U.S. application Ser. No. 12/941,974, filed Nov. 8,2010, now U.S. Pat. No. 8,681,757, which claims the benefit of earlierfiling date and right of priority to Korean Application No.10-2010-0046256, filed on May 18, 2010, and also claims the benefit ofU.S. Provisional Application Ser. No. 61/259,576, filed on Nov. 9, 2009,and 61/285,917, filed on Dec. 11, 2009, the contents of which are allhereby incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communication, and moreparticularly, to a method of transmitting a PLCP frame in a WLAN systemand a wireless apparatus supporting the method.

Related Art

With the recent development of information communication technology, avariety of wireless communication techniques are being developed. Fromamong them, a WLAN is a technique which enables wireless access to theInternet at homes or companies or in specific service providing areasthrough mobile terminals, such as a Personal Digital Assistant (PDA), alaptop computer, and a Portable Multimedia Player (PMP), on the basis ofradio frequency technology.

Since Institute of Electrical and Electronics Engineers (IEEE) 802(i.e., the standard organization of WLAN technology) was set up onFebruary, 1980, lots of standardization tasks are being performed.

The initial WLAN technology was able to support the bit rate of 1 to 2Mbps through frequency hopping, band spreading, and infraredcommunication by using a 2.4 GHz frequency band in accordance with IEEE802.11, but the recent WLAN technology can support a maximum bit rate of54 Mbps by using Orthogonal Frequency Division Multiplex (OFDM). Inaddition, in the IEEE 802.11, the standardization of various techniques,such as the improvements of Quality of Service (QoS), the compatibilityof Access Point (AP) protocols, security enhancement, radio resourcemeasurement, wireless access vehicular environment for vehicleenvironments, fast roaming, a mesh network, interworking with anexternal network, and wireless network management, is put to practicaluse or being developed.

Furthermore, as a technique for overcoming limits to the communicationspeed considered as vulnerabilities in the WLAN, there is IEEE 802.11nwhich has recently been standardized. The object of the IEEE 802.11n isto increase the speed and reliability of a network and to expand thecoverage of a wireless network. More particularly, the IEEE 802.11n isbased on a Multiple Inputs and Multiple Outputs (MIMO) technique usingmultiple antennas on both sides of a transmitter and a receiver in orderto support a High Throughput (HT) having a data processing speed of 540Mbps or higher, minimize transmission errors, and optimize the datarate. Further, the IEEE 802.11n may use not only a coding method oftransmitting several redundant copies in order to increase datareliability, but also an Orthogonal Frequency Division Multiplex (OFDM)method in order to increase the data rate.

In addition to a PLCP format supporting legacy STAs, an HT green fieldPLCP format (that is, a PLCP format efficiently designed for HT STAs)which can be used in a system composed of HT STAs supporting IEEE802.11n has been introduced into the IEEE 802.11n HT (High Throughput)WLAN system. Furthermore, the IEEE 802.11n HT (High Throughput) WLANsystem supports an HT mixed PLCP format which is a PLCP format designedto support an HT system in a system in which legacy STAs and HT STAscoexist.

In the HT mixed PLCP frame, an HT-SIG field is subjected to encoding andinterleaving processes and then mapped for modulation. Here, a QBPSKconstellation is used. The QBPSK constellation is a constellationshifted from a BPSK constellation by 90°. An HT-SIG field can be simplydetected because an L-SIG field uses a common BPSK constellation.

For detailed information about the HT green field PLCP format and the HTmixed PLCP format, reference can be made to “IEEE P802.11n™/D11.0, DraftSTANDARD for Information Technology-Telecommunications and InformationExchange Between Systems-Local and Metropolitan Area Networks-SpecificRequirements Part 11: Wireless LAN Medium Access Control (MAC) andPhysical Layer (PHY) Specifications Amendment 5: Enhancements for HigherThroughput, Clause 20. High Throughput PHY specification” disclosed onJune, 2009.

In IEEE 802.11n, 8 bits for CRC check are allocated to the HT-SIG field,thereby being capable of protecting 0-33 bits (0-23 bits are an HT-SIG1field and 0-9 bits are an HT-SIG2 field) from among 48 bits. In the CRCoperation, after a shift register is set to an initial value, input bitsare sequentially calculated through the shift register, and the last bitenters the shift register. After all operations are finished, bitsremaining in the shift register are obtained as outputs. For example,assuming that (m0 . . . m33)={1 1 1 1 0 0 0 1 0 0 1 0 0 1 1 0 0 0 0 0 00 0 0 1 1 1 0 0 0 0 0 0 0}, CRC bits {c7 . . . c0}={1 0 1 0 1 0 0 0}.

When an HT STA detects the HT-SIG field of an HT mixed PLCP frame, twokinds of operations are possible in addition to a mode in which theHT-SIG field is normally read and operated. The HT STA can be operatedin the legacy mode because it has recognized that the HT-SIG field isnot the HT-SIG field or, although it has recognized that the HT-SIGfield is the HT-SIG field, can inform CRC error throughPHY-RXEDN.indication (Format Violation) without transmittingPHY-RXSTART.indication because of errors detected as the result of theCRC execution. At this time, the HT PHY terminal maintainsPHY-CCA.indication (BUSY, channel-list) until a received level dropsless than a specific CCA sensitivity level (e.g., an energy detectionthreshold) indicating an idle channel.

In each of the OFDM symbols of a 20 MHz channel of IEEE 802.11n, foursubcarriers are composed of a pilot signal. This is for coherentdetection robust to frequency offset and phase noise. The pilot signalcan be modulated into a BPSK constellation, placed in indices −21, −7,7, 21, and represented by {0,0, . . . , 0,1, 0, . . . , 0,1, 0, . . . ,0,1, 0, . . . , 0, −1, 0, 0}. Meanwhile, the pilot subcarriers arescrambled by a sequence Pn.

With the WLAN being widely spread and applications using the WLANbecoming diverse, a need for a new WLAN system capable of supporting ahigher throughput than the data processing speed supported by the IEEE802.11n is recently gathering strength. A Very High Throughput (VHT)WLAN system is one of the IEEE 802.11 WLAN systems which have recentlybeen proposed in order to support a data processing speed of 1 Gbps orhigher.

In IEEE 802.11 TGac in which the standardization of a VHT WLAN system isbeing carried out, in order to provide the throughput of 1 Gbps orhigher, research is being done on a scheme using 8×8 MIMO and a channelbandwidth of 80 MHz or higher and a PLCP format for efficientlysupporting each STA in a WLAN system in which a legacy STA, an HT STA,and a VHT STA coexist. As part of improving the performance of anSU-MIMO mode introduced in IEEE 802.11n and an MU-MIMO mode to be newlyintroduced into the VHT WLAN system, a method of configuring a PLCPframe format capable of effectively supporting the SU-MIMO mode and theMU-MIMO mode and guaranteeing the coexistence by preventing themalfunction of the legacy STA and the HT STA, and a wireless apparatussupporting the method need to be taken into consideration.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of configuringa PLCP frame which is capable of improving the performance of SU-MIMOand of supporting MU-MIMO in a WLAN system in which an L STA, an HT STA,and a VHT STA coexist and an apparatus supporting the method.

Another object of the present invention is to provide a PLCP frameformat which is capable of reducing preamble overhead and alsopreventing the malfunction of an HT STA in a WLAN system in which alegacy STA, the HT STA, and a VHT STA coexist.

In an aspect of the present invention, a method of transmitting aPhysical Layer Convergence Procedure (PLCP) frame in a Wireless LocalArea Network (WLAN) system is provided, the method includes generating aMAC Protocol Data Unit (MPDU) to be transmitted to a destination station(STA), generating a PLCP Protocol Data Unit (PPDU) by adding a PLCPheader, comprising an L-SIG field containing control information for alegacy STA and a VHT-SIG field containing control information for a VHTSTA, to the MPDU, and transmitting the PPDU to the destination STA,wherein a constellation applied to some of Orthogonal Frequency DivisionMultiplex (OFDM) symbols of the VHT-SIG field is obtained by rotating aconstellation applied to an OFDM symbol of the L-SIG field.

The L-SIG field may be transmitted as one OFDM symbol, and the OFDMsymbol of the L-SIG field may be mapped to a BPSK constellation.

The OFDM symbols of the VHT-SIG field may include two OFDM symbols ofVHT-SIG1 and VHT-SIG2, and the VHT-SIG1 symbol may be modulated by usingthe same method and constellation as the OFDM symbol of the L-SIG field.

The L-SIG field may be transmitted as one OFDM symbol, the OFDM symbolof the L-SIG field may be mapped to a BPSK constellation, and theVHT-SIG field may be transmitted as two OFDM symbols of VHT-SIG1 andVHT-SIG2, the OFDM symbol of the VHT-SIG1 may be mapped to a BPSKconstellation and the OFDM symbol of the VHT-SIG2 is mapped to a QBPSKconstellation.

The OFDM symbols of the VHT-SIG field may include three OFDM symbols ofVHT-SIG1, VHT-SIG2, and VHT-SIG3, and the VHT-SIG1 symbol may bemodulated by using the same method and constellation as the OFDM symbolof the L-SIG field.

The OFDM symbol of the VHT-SIG field may include a VHT pilot signal, andthe VHT pilot signal may be mapped to a constellation obtained byrotating a constellation applied to a pilot signal included in the OFDMsymbol of the L-SIG field.

The pilot signal included in the OFDM symbol of the L-SIG field may bemapped to a BPSK constellation.

The VHT pilot signal may be mapped to a BPSK constellation rotated 180°.

In another aspect of the present invention, a method of configuring aPLCP frame in a WLAN system is provided, the method includes generatingan MPDU to be transmitted to a destination STA, generating a PPDU byadding a PLCP header, comprising an L-SIG field containing controlinformation for a legacy STA and a VHT-SIG field containing controlinformation for a VHT STA, to the MPDU, and transmitting the PPDU to thedestination STA, wherein an L-CRC bit string used in a Cyclic RedundancyCheck (CRC) of the destination STA and included in the L-SIG field and aVHT-CRC bit string the CRC of the destination STA and included in theVHT-SIG field are obtained on the basis of different CRC polynomials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the PHY layer architecture of IEEE 802.11;

FIG. 2 is a block diagram showing an example of a VHT mixed PLCP frameformat which supports the SU-MIMO mode in a WLAN system in which anL-STA, an HT-STA, and a VHT-STA coexist;

FIG. 3 is a block diagram showing an example of a VHT mixed PLCP frameformat which supports the MU-MIMO mode in a WLAN system in which anL-STA, an HT-STA, and a VHT-STA coexist;

FIG. 4 is a diagram showing constellations which are used in an L-SIGfield and an HT-SIG field and an example of a constellation which can beused in a VHT-SIG field according to the present invention;

FIG. 5 is a diagram showing constellations which are used in an L-SIGfield and an HT-SIG field and another example of a constellation whichcan be used in a VHT-SIG field according to the present invention;

FIG. 6 is a block diagram showing a VHT mixed PLCP frame formatsupporting the SU-MIMO mode according to an embodiment of the presentinvention;

FIG. 7 is a block diagram showing a VHT mixed PLCP frame formatsupporting the MU-MIMO mode according to an embodiment of the presentinvention;

FIG. 8 shows an example of constellations for symbols and constellationsfor pilots, of each field of a PLCP frame according to an embodiment ofthe present invention; and

FIG. 9 is a block diagram of a wireless apparatus in which an embodimentof the present invention is implemented.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Some embodiments of the present invention are described in detail belowwith reference to the accompanying drawings.

A WLAN (wireless local area network) system in which the embodiments ofthe present invention are implemented includes at least one BasicService Set (BSS). The BSS is a set of stations (STAs) successfullysynchronized for communication. The BSS can be classified into anIndependent BSS (IBSS) and an Infrastructure BSS.

The BSS includes at least one STA and an Access Point (AP). The AP is amedium providing connection through the wireless medium of each STAwithin the BSS. The AP can be called another terminology, such as acentralized controller, a Base Station (BS), or a scheduler.

An STA is a certain function medium including a MAC (medium accesscontrol) and PHY (wireless-medium physical layer) interface whichsatisfies the IEEE 802.11 standard. The STA can be an AP or a non-APSTA, but refers to a non-AP STA unless described otherwise. The STA canbe called another terminology, such as User Equipment (UE), a MobileStation (MS), a Mobile Terminal (MT), a portable device, or an interfacecard.

STAs can be classified into a VHT-STA, an HT-STA, and a Legacy (L)-STA.The HT-STA refers to an STA supporting IEEE 802.11n, and the L-STArefers to an STA supporting a lower version of IEEE 802.11n (e.g., IEEE802.11a/b/g). The L-STA is called a non-HT STA.

FIG. 1 is a diagram showing the PHY layer architecture of IEEE 802.11.

The PHY layer architecture of IEEE 802.11 consists of a PHY LayerManagement Entity (PLME), a Physical Layer Convergence Procedure (PLCP)sublayer 110, and a Physical Medium Dependent (PMD) sublayer 100. ThePLME provides a function of managing the physical layer in cooperationwith a MAC Layer Management Entity (MLME). The PLCP sublayer 110transfers a MAC Protocol Data Unit (MPDU), received from a MAC sublayer120, to the PMD sublayer 100 or a frame, received from the PMD sublayer100, to the MAC sublayer 120 between the MAC sublayer 120 and the PMDsublayer 100 according to the instruction of the MAC layer 120. The PMDsublayer 100 is a lower layer of the PLCP, and it enables thetransmission and reception of a physical layer entity between two STAsthrough a radio medium.

In a process of receiving an MPDU from the MAC sublayer 120 andtransferring the MPDU to the PMD sublayer 100, the PLCP sublayer 110adds a supplementary field, including information necessary for aphysical layer transceiver, to the MPDU. The supplementary field can bea PLCP preamble in the MPDU, a PLCP header, or tail bits necessary for adata field. The PLCP preamble functions to synchronize a receiver and tomake the receiver prepare for antenna diversity, before a PLCP ServiceData Unit (PSDU) (=MPDU) is transmitted to the receiver. The PLCP headerincludes a field including information about a frame. The PLCP isdescribed in more detail later with reference to FIG. 2.

The PLCP sublayer 110 generates a PLCP Protocol Data Unit (PPDU) byadding the above-described field to the MPDU and transmits the PPDU to areception station via the PMD sublayer 100. The reception stationreceives the PPDU, obtains information necessary for data restorationfrom a PLCP preamble and a PLCP header, and restores data on the basisof the obtained data.

FIG. 2 is a block diagram showing an example of a VHT mixed PLCP frameformat which supports the SU-MIMO mode in a WLAN system in which anL-STA, an HT-STA, and a VHT-STA coexist.

The VHT mixed PLCP frame can include an L-STF field 210, an L-LTF field220, an L-SIG field 230, an HT-SIG field 240, a VHT-SIG field 250, a VHTSTF field 260, data VHT-LTF fields 270, an extension VHT-LTF field 280,and a data field 290.

The PLCP sublayer transforms an MPDU, received from an MAC layer, intothe data field 290 of FIG. 2 by adding necessary information to theMPDU, generates a PPDU frame 200 by adding fields, such as the L-STFfield 210, the L-LTF field 220, the L-SIG field 230, the HT-SIG field240, the VHT-SIG field 250, the VHT STF field 260, the data VHT-LTFfields 270, and the extension VHT-LTF field 280, to the data field 290,and transmits the PPDU frame 200 to one or more STAs through the PMDlayer.

The L-STF field 210 is used for frame timing acquisition, Automatic GainControl (AGC) control, coarse frequency acquisition, and so on.

The L-LTF field 220 is used in channel estimation for the demodulationof the L-SIG field 230, the HT-SIG field 240, and the VHT-SIG field 250.

The fields up to the VHT-SIG field 250 are not subjected to beam-formingand transmitted so that they can be received and recognized by all STAsincluding an L-STA. The fields transmitted after the VHT-SIG field 250,such as the VHT STF field 260, the data VHT-LTF fields 270, theextension VHT-LTF field 280, and the data field 290, can be subjected toprecoding and beam-forming and then transmitted.

The VHT-STF field 260 is used for a VHT-STA to improve AGC estimationand for an STA, receiving the VHT-STF field 260, to take a portion wheretransmission power is varied because of precoding into consideration.

The plurality of data VHT-LTF fields 270 is used for channel estimationfor the demodulation of the data field 290. Additionally, the extensionVHT-LTF field 280 for channel sounding can be used.

A Short Training Field (STF), such as the L-STF field 210 and the VHTSTF field 260, is used for frame timing acquisition, AGC control, and soon and thus also called a synchronization signal or a synchronizationchannel. That is, the STF is used to synchronize STAs or an STA and anAP.

A Long Training Field (LTF), such as the L-LTF field 220 and the dataVHT-LTF fields 270, is used for channel estimation for the demodulationof data or control information or both and thus also called a referencesignal, a training signal, or a pilot.

The L-SIG field 230, the HT-SIG field 240, and the VHT-SIG field 250provide various pieces of information necessary for the demodulation anddecoding of data and thus also called control information.

The VHT SIG field 250 can include at least one of fields listed in Table1, for example.

TABLE 1 Field Name Description MU-MIMO Indicator Indicate whetherMU-MIMO is used or can toggle SU-MIMO/MU-MIMO. Bandwidth Indicate thebandwidth of a channel VHT Length Indicate the number of data octets ofa PSDU STA Indicator Indicate a receiving STA. Indicate the address ofan STA or ID information about an STA, such as AID Multiplexing NumberIndicate the number of STAs (or users) multiplexed by MU-MIMO DecodingIndicator Indicate information for decoding data MCS Indicate Modulationand Coding Scheme (MCS) information necessary to decode data Short GIIndicate whether a short Guard Interval (GI) is used. Number ofExtension Indicate the number of extension spatial streams SpatialStreams CRC Indicate a check value for checking whether there is errorin transmitted data Tail Bits Used in the trellis termination of aconvolution coder

In Table 1, the field names are only illustrative, and names differentfrom the above field names can be used. The fields of Table 1 are onlyillustrative. For example, some of the fields in Table 1 can be omitted,and other fields can be further added to the fields of Table 1.Furthermore, the PPDU frame according to the PPDU frame formatillustrated in FIG. 2 is generated in the PLCP sublayer of an STA andtransmitted to a transmission destination STA via the PMD sublayer. Someof the fields of the PPDU frame of FIG. 2 can be omitted, or otherfields can be added to the fields of the PPDU frame of FIG. 2.

FIG. 3 is a block diagram showing an example of a VHT mixed PLCP frameformat which supports the MU-MIMO mode in a WLAN system in which anL-STA, an HT-STA, and a VHT-STA coexist.

The VHT mixed PLCP frame can include an L-STF field 310, an L-LTF field320, an L-SIG field 330, an HT-SIG field 340, a plurality of VHT-SIGfields 350, a VHT STF field 360, data VHT-LTF fields 370, extensionVHT-LTF fields 380, and a data field 390.

The function of each of the fields is the same as that of FIG. 2. Unlikethe VHT mixed PLCP frame format of FIG. 2, the VHT mixed PLCP frameformat of FIG. 3, supporting the MU-MIMO mode in which data istransmitted to a plurality of destination STAs at the same time, has theplurality of VHT-SIG fields 350.

FIG. 3 illustrates a case where an AP performs MU-MIMO transmission toan STA1 and an STA2. In FIG. 3, two VHT-SIG fields (i.e., a VHT-SIG1field 350-1 and a VHT-SIG2 field 350-2) are used. The VHT-SIG1 field350-1 and the VHT-SIG2 field 350-2 include control information about theSTA1 and the STA2, respectively. That is, the PLCP frame formatsupporting the MU-MIMO mode can have the same number of VHT-SIG fieldsas destination STAs.

Furthermore, in FIG. 3, the AP transmits data to the STA 1 by usingthree spatial streams 390-1 and data to the STA 2 by using two spatialstreams 390-2. FIG. 3 illustrates a case where a total of five spatialstreams are used. In this case, a VHT-LTF field can be included in eachof the spatial streams, and thus the data VHT-LTF fields 370 can consistof five VHT-LTFs fields in FIG. 3.

As can be seen from the VHT mixed PLCP frame formats shown in FIGS. 2and 3, in general, the L-SIG field 330 for the L-STA and the HT-SIGfield 340 for the HT-STA are transmitted earlier than the VHT-SIG field350 for a VHT-STA in order to support backward compatibility with anL-STA and an HT-STA.

Here, the operations of the STAs are described below. The L-STA readsthe L-SIG field 330 from the received PLCP frame and performs detectionassuming that the received PLCP frame is its own data packet. However,the L-STA has received the PLCP frame transmitted in a data format foran HT STA, including the HT-SIG field which is not data that can beaccepted (or recognized) by the L-STA. Consequently, if CRC check isperformed on a result of demodulation and decoding performed assumingthat the PLCP frame has a format for the L-STA, error occurs. Such anoperation can occur even in the case of the HT-STA. Like the L-STA, theHT-STA reads the L-SIG field and the HT-SIG field, recognizes fieldssubsequent to the VHT-SIG field, transmitted after the HT-SIG field, asthe HT-LTF field, etc., assuming that data is transmitted in a form forthe HT STA, and receives the fields. Consequently, a result ofdemodulation and decoding for the PLCP frame has error in a result ofCRC check.

However, in order to satisfy backward compatibility, to alwaysconsecutively transmit the three SIG fields (i.e., the L-SIG field, theHT-SIG field, and the VHT-SIG field) in the VHT mixed PLCP frame asdescribed above is not preferred in that it increases preamble overhead.If, in order to reduce preamble overhead, the three SIG fields are notconsecutively transmitted and the HT-SIG field of the three SIG fieldsis not transmitted (in other words, only the L-SIG field and the VHT-SIGfield are transmitted), preamble overhead of 8 μs can be reduced. Here,in case where the HT-SIG field including control information for theHT-STA is not transmitted in order to reduce preamble overhead, there isa possibility that the HT-STA can malfunction. Accordingly, there is aneed for a solution for such malfunction.

There is proposed a method of configuring a VHT mixed PLCP frame andsetting up fields, wherein an HT-STA which has received the VHT mixedPLCP frame in which only an L-SIG field and a VHT-SIG field except anHT-SIG field are transmitted in order to reduce preamble overhead cannormally recognize that the VHT mixed PLCP frame is not its own datapacket without malfunction.

According to an embodiment of the method of configuring the VHT mixedPLCP frame and setting up the fields, proposed by the present invention,CRC bits included in the VHT-SIG field are set up such that a result ofCRC error is obtained when an HT-STA performs CRC check on the CRC bits.

For example, a CRC polynomial used for the CRC check can be set updifferently from a CRC polynomial for the HT-SIG field, and the value ofCRC bits included in the VHT-SIG field can be determined by performingCRC calculation on the basis of the different CRC polynomial.

An AP transmits a PLCP frame, including the L-SIG field and the VHT-SIGfield, except the HT-SIG field. Here, the VHT-SIG field can include CRCbits, obtained on the basis of a new CRC polynomial, as a subfield. AnHT-STA which has received the VHT mixed PLCP frame according to anembodiment of the present invention from which the HT-SIG field has beenomitted obtains a result of CRC error as the result of CRC checkperformed on the basis of a conventional CRC polynomial used by theHT-STA. Accordingly, the HT-STA can know that the VHT mixed PLCP frameis not data having a format for the HT-STA.

According to another embodiment of the present invention in which anHT-STA which has received a VHT mixed PLCP frame obtains a result of CRCerror as the result of CRC check on an HT-SIG field, a CRC polynomialfor the CRC check on the HT-SIG field is used, but the CRC bits of aVHT-SIG field are obtained by performing an XOR (i.e., exclusive OR)operation of CRC parity bits and a specific bit pattern (hereinafter,referred to as ‘CRC masking’). The VHT-SIG field, including the CRC bitsobtained by the CRC masking, generates CRC error to the HT-STA, but theVHT-STA can be normally operated.

In this case, an identifier (ID) for identifying an STA, such as an STAID or an Association ID (AID) of the VHT-STA, can be used as thespecific bit pattern subjected to the XOR operation with the CRC paritybits. The HT-STA which has received the VHT-SIG field, including the CRCbits obtained by CRC masking performed on the AID, recognizes the CRCerror as a simple CRC error because it does not know whether the CRCmasking has been performed and thus can know that the VHT-SIG field isnot data having a format for the HT-STA.

The VHT-SIG field is set up such that an HT-STA which has received theVHT-SIG field according to another embodiment of the present inventionrecognizes the VHT-SIG field as having a data format for an L-STA andoperates.

In a method of setting up the VHT-SIG field according to an embodimentof the present invention, a constellation for the VHT-SIG field is newlydefined and used so that the HT-STA which has received the VHT-SIG fieldrecognizes the VHT-SIG field as having a data format for an L-STA andoperates.

FIG. 4 is a diagram showing constellations which are used in an L-SIGfield and an HT-SIG field, respectively, and an example of aconstellation which can be used in a VHT-SIG field according to thepresent invention.

Referring to FIG. 4, an L-STA reads the L-SIG field 410 composed of one4 μs SIG field, and an HT-STA read the HT-SIG field 420 composed of a 4μs HT-SIG1 field 420-1 and a 4 μs HT-SIG2 field 420-2. The constellationused in the L-SIG field 410 as in FIG. 4 is BPSK, and the HT-SIG1 field420-1 and the HT-SIG2 field 420-2, each having 24 bits, are subjected toquadrature BPSK (QBPSK) modulation.

The constellation of the VHT-SIG field 430 shown in FIG. 4 is an exampleof the constellation of the VHT-SIG field according to an embodiment ofthe present invention. The VHT-SIG field 430 according to the embodimentof the present invention can be composed of a VHT-SIG1 field 430-1 and aVHT-SIG2 field 430-2. BPSK is used as a modulation method applied to theVHT-SIG field, but the VHT-SIG field 430 can be transmitted as acombination of BPSK constellations differently rotated per VHT-SIG OFDMsymbol.

The example of FIG. 4 shows an example in which BPSK is applied to theconstellation of the first OFDM symbol VHT-SIG1 430-1 of the VHT-SIGfield 430 and a BPSK constellation rotated 90° is applied to theconstellation of the second OFDM symbol VHT-SIG2 430-2 of the VHT-SIGfield 430. If the above constellations are used, an HT-STA reads aVHT-SIG field and does not recognize the read VHT-SIG field as HT-SIGfield. In order for the HT-STA to recognize the VHT-SIG field as HT-SIGfield, it is required that the VHT-SIG1 430-1 and the VHT-SIG2 430-2 besubjected to QBPSK modulation or at least VHT-SIG1 430-1 be subjected toQBPSK modulation although it is determined whether an HT-SIG field is anHT-SIG field by using only the VHT-SIG1 430-1 according to an example ofa method of implementing an STA. However, the VHT-STA can know that areceived PLCP frame format has a format for the VHT-STA through therotated constellation of the VHT-SIG2 430-2 because it already knowswhether a constellation has been rotated and the degree of rotation.

FIG. 5 is a diagram showing constellations which are used in an L-SIGfield and an HT-SIG field, respectively, and another example of aconstellation which can be used in a VHT-SIG field according to thepresent invention.

In a VHT WLAN system, QPSK, 16QAM, 64QAM, and control information bitsnecessary to support 256QAM and eight or more spatial streams can begreater than 48 bits. In case where 1/2 code-rate channel encodingapplied to an HT-SIG field is used in order to make a maximum thetransmission coverage of control information, the number of OFDM symbols(when 20 MHz and 24 subcarriers are used) is three. In this case, aVHT-SIG field 530 can include three OFDM symbols (i.e., VHT-SIG1 530-1,VHT-SIG2 530-2, and VHT-SIG3 530-3.

As in the example of FIG. 4, BPSK is used as a modulation method appliedto the VHT-SIG field 530, but the VHT-SIG field 530 can be transmittedas a combination of BPSK constellations differently rotated per VHT-SIGOFDM symbol. The degree of rotation can have a different angle for eachsymbol, and some of the degrees of rotation can have the same value. Theexample of FIG. 5 shows a case where a BPSK modulation method has beenapplied to the VHT-SIG1 530-1 and the VHT-SIG2 530-2 and a QBPSKmodulation method has been applied to the VHT-SIG3 530-3. An HT-STA doesnot recognize the VHT-SIG1 530-1 and the VHT-SIG2 530-2 as having a PLCPframe format for the HT-STA because the VHT-SIG1 530-1 and the VHT-SIG2530-2 have not been subjected to QBPSK modulation. However, the VHT-STAcan know that a received PLCP frame format has a format for the VHT-STAthrough the rotated constellations of the VHT-SIG2 530-2 or the VHT-SIG3530-3 or both because it already know whether the constellations havebeen rotated and the degree of rotation.

FIGS. 6 and 7 are block diagrams showing a VHT mixed PLCP frame formatsupporting the SU-MIMO mode and a VHT mixed PLCP frame format supportingthe MU-MIMO mode, respectively, according to an embodiment of thepresent invention.

It can be seen that unlike the VHT mixed PLCP frame format of FIG. 2,the VHT mixed PLCP frame format of FIG. 6 has an HT-SIG field omittedbetween an L-SIG field 630 and a VHT-SIG field 650. It can also be seenthat unlike the VHT mixed PLCP frame format of FIG. 3, the VHT mixedPLCP frame format of FIG. 7 has an HT-SIG field omitted between an L-SIGfield 730 and VHT-SIG fields 750.

A possible malfunction of an HT-STA which has received the VHT mixedPLCP frame from which the HT-SIG field has been omitted according to theembodiment of the present invention can be solved by using the method ofgenerating CRC error through the set-up of CRC bits included in theVHT-SIG field and the method of configuring the VHT-SIG field as acombination of the rotated BPSK constellations. The methods can be usedindependently or together.

In the method of configuring the VHT-SIG field as a combination of therotated BPSK constellations, a VHT-SIG symbol is configured by using twoor three BPSK constellations for an L-STA or a first VHT-SIG symbolVHT-SIG1 is maintained to constellation BPSK of an L-SIG field, and thena second VHT-SIG symbol VHT-SIG2 transmitted after the first VHT-SIGsymbol VHT-SIG1 (i.e., a VHT-SIG2 and a VHT-SIG3 in case where theVHT-SIG field is composed of three symbols) is configured by using arotated L-SIG constellation (i.e., BPSK constellation). This is becausein order for an HT-STA not to malfunction when receiving a VHT mixedPLCP frame from which an HT-SIG field having a format, such as that ofFIG. 6 or 7, has been omitted, the first VHT-SIG symbol VHT SIG1 has tobe maintained to the constellation of an L-SIG symbol. Here, aconstellation rotated 180.degree. from the constellation of an L-SIGsymbol can be used as the first VHT-SIG symbol VHT SIG1. Even in thiscase, an HT-STA can determine that the symbol has a PLCP frame formatfor (or supporting) an L-STA because a constellation of the symbol hasnot been shifted by 90° and operate.

According to another embodiment of the present invention, themalfunction of an HT-STA can be prevented by using a pilot included inthe OFDM symbol of a VHT-SIG field. A VHT-STA can distinguish SIG fieldsby using a pilot or can distinguish SIG fields by using a pilot anddetermining whether a symbol constellation of the SIG field has beenrotated.

Four subcarriers in each OFDM symbol of a channel having a bandwidth of20 MHz consist of a pilot signal. The pilot signal can be modulated intoa BPSK constellation and placed in indices −21, −7, 7, and 21.

$\begin{matrix}{P_{({i_{STS},n})}^{28,28} = \left\{ {0,0,0,0,0,0,0,\Psi_{i_{STS}{n \odot 4}}^{(N_{STS})},0,0,0,0,0,0,0,0,0,0,0,0,0,\Psi_{{i_{STS}{({n + 1})}} \oplus 4}^{(N_{STS})},0,0,0,0,0,0,0,0,0,0,0,0,0,\Psi_{{i_{STS}{({n + 3})}} \oplus 4}^{(N_{STS})},0,0,0,0,0,0,0,0,0,0,0,0,0,\Psi_{{i_{STS}{({n + 3})}} \oplus 4}^{(N_{STS})},0,0,0,0,0,0,0} \right\}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Equation 1 represents a pilot sequence for an n^(th) symbol and ani_(STS) ^(th) Space Time Stream (STS). Here, n indicates a symbolnumber, ⊕ indicates a modulo operator, and n⊕a is the remainder obtainedby dividing n by a. A pattern of a pilot Ψ_(i) _(STS) _(,n) ^((N) ^(STS)⁾ is defined as in Table 2.

TABLE 2 N_(STS) i_(STS)

1 1 1 1 1 −1 2 1 1 1 −1 −1 2 2 1 −1 −1 1 3 1 1 1 −1 −1 3 2 1 −1 1 −1 3 3−1 1 1 −1 4 1 1 1 1 −1 4 2 1 1 −1 1 4 3 1 −1 1 1 4 4 −1 1 1 1

According to an embodiment of the present invention, a BPSKconstellation used for a pilot is used as a rotated BPSK constellationin case where the BPSK constellation is applied to a pilot for a VHT SIGfield. For example, a BPSK constellation rotated 180.degree. can beapplied to a VHT-SIG field. The degree of rotation can be set up invarious ways, and the present invention is not limited to the degree ofrotation and the number of pilots included in each OFDM symbol. OFDMsymbols in a channel having a bandwidth of 20 MHz including four pilotsare described below as an example. However, the technical spirit of thepresent invention can also be applied to a case where a channel having achannel bandwidth of 40 MHz, 80 MHz, or 80 MHz or higher, composed ofsubcarriers 6 or more of which include a pilot in an OFDM symbol.

According to embodiments, a receiving STA can know the type of an SIGfield by using one pilot, from among symbols constituting the SIG field,or can accurately know the type of an SIG field by using a pilot of twoor three symbols constituting the SIG field.

A method of distinguishing the types of SIG fields by using a pilotaccording to an embodiment of the present invention can be performed asfollows.

In the case in which a BPSK constellation rotated 180.degree. is used asa constellation applied to a pilot according to an embodiment of thepresent invention, the pilot can be represented by using two kinds ofmethods.

The first method is a method of rotating a pilot itself 180°. A pilotproposed by the present invention can use {0,0, . . . , 0,−1,0, . . .0,−1,0, . . . , 0,−1,0, . . . , 0,1,0, . . . , 0} in which pilot valuesplaced at indices −21, −7, 7, and 21 have been changed from 1 to −1 andfrom −1 to 1, which is a result of the pilot itself rotated 180°.

The second method is a method of rotating a scrambling sequence 180°. Inthis method, scrambling is performed by using a 180.degree. rotatedscrambling sequence which can be conventionally obtained by multiplyingthe scrambling sequence by −1.

In the method of a receiving STA distinguishing SIG fields by using apilot of an OFDM symbol, the receiving STA can distinguish the SIGfields by using a correlation of pilots or a phase difference between anLTF field and a pilot.

FIG. 8 shows an example of constellations for symbols and constellationsfor pilots, of each field of a PLCP frame according to an embodiment ofthe present invention.

The example of FIG. 8 corresponds to another embodiment of the presentinvention described with reference to FIG. 4. From FIG. 8, it can beseen that the constellation 835 of an L-SIG field 830 is used as thesymbol constellation 855-1 of a VHT-SIG1 symbol 850-1, and theconstellation 835 of the L-SIG field 830 which has been rotated is usedas the symbol constellation 855-2 of a VHT-SIG2 symbol 850-2.

The pilot of the present invention exists in the SIG field and the datafield. In marking the pilot sequence in FIG. 8, {0,0, . . . , 0,1,0, . .. , 0,1,0, . . . , 0,1,0, . . . , 0,−1,0, . . . , 0} is simplyrepresented by using only the pilot values placed at the indices −21,−7, 7, and 21 in the form of with 0 being omitted.

It can be seen that the pilot constellation 857-1 of the VHT SIG1 symbol850-1 and the symbol constellation 857-2 of the VHT SIG2 symbol 850-2have been rotated 180.degree., as compared with the pilot constellation837 of the L-SIG field 830.

An HT-STA which has received a VHT mixed PCLP frame, such as that shownin the example of FIG. 8, recognizes that the PLCP frame received fromthe constellation 835 of the L-SIG field 830 and the constellation 855-1of the VHT-SIG1 symbol 850-1 has a frame format supporting an L-STA andthen operates. Furthermore, an HT-STA which has received a VHT mixedPCLP frame can recognize that the PLCP frame received from the pilotconstellation 857-1 of the VHT SIG1 symbol 850-1, rotated 180.degree.,and the symbol constellation 857-2 of the VHT SIG2 symbol 850-2 does nothave a frame format for the HT-STA.

FIG. 9 is a block diagram of a wireless apparatus in which an embodimentof the present invention is implemented. The wireless apparatus 900 canbe a part of a non-AP STA or an AP.

The wireless apparatus 900 includes a frame generation unit 910 and aframe transmission unit 920. The frame generation unit 910 generates theVHT mixed PLCP frame according to the above-described embodiments. Here,the CRC bits of a VHT SIG field included in the PLCP header of the VHTmixed PLCP frame can be set up such that they generate error as theresult of CRC check performed by an HT-STA which has received the VHTmixed PLCP frame. Furthermore, the constellation or the pilot of theVHT-SIG field can be set up by using the above-described methods. Theframe transmission unit 920 transmits the generated VHT mixed PLCP frameto one or more wireless apparatuses.

The frame generation unit 910 and the frame transmission unit 920 can beimplemented in one chip in the form of a processor. Each of theembodiments in which a frame is generated can be implemented in asoftware module, stored in memory, and executed by a processor.

According to an embodiment of the present invention, an SU-MIMO mode andan MU-MIMO mode can be effectively supported, preamble overhead can bereduced, and at the same time the malfunction of a legacy STA and an HTSTA can be prevented. Accordingly, the coexistence of a VHT STA, thelegacy STA, and the HT STA can be guaranteed.

While the invention has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. An apparatus, comprising: at least one processor;at least one memory including computer program code, the memory and thecomputer program code configured to, working with the processor, causethe apparatus to perform at least the following: receive a data frame,the data frame comprising: a legacy signal field including a firstsymbol that is mapped onto a first binary phase shift keying (BPSK)constellation; a Very High Throughput Signal (VHT-SIG) field including asecond symbol and a third symbol, the second symbol being mapped ontothe first BPSK constellation, and the third symbol being mapped onto asecond BPSK constellation, and the second BSPK constellation beingrotated by 90° counter-clockwise relative to the first BPSKconstellation; and wherein the second symbol includes an indicatorindicating Single User-Multiple Input Multiple Output (SU-MIMO)transmission or multi user-MIMO (MU-MIMO) transmission, and the thirdsymbol includes a cyclic redundancy check (CRC) for the second symboland, at least a portion of, the third symbol.
 2. The apparatus of claim1, further comprising a wireless local area network transceiver, whereinthe data frame is received by the wireless local area networktransceiver.
 3. The apparatus of claim 2, further comprising at leastone antenna, wherein the wireless local area network transceiverreceives the data frame by way of the at least one antenna.
 4. Theapparatus of claim 1, wherein the third symbol is immediately after thesecond symbol.
 5. The apparatus of claim 1, wherein the data frame is aPhysical Layer Convergence Procedure Protocol Data Unit (PPDU).
 6. Theapparatus of claim 4, wherein the PPDU omits a high throughput signal(HT SIG) field between the legacy field and the VHT-SIG field.
 7. Theapparatus of claim 1, wherein the VHT-SIG field further includes amultiplexing number indicating a number of spatial streams.
 8. Theapparatus of claim 7, wherein the second symbol includes themultiplexing number indicating the number of spatial streams.
 9. Theapparatus of claim 1, wherein the VHT-SIG field is immediately after thelegacy field.
 10. The apparatus of claim 1, wherein the second symbol isimmediately after the first symbol.
 11. A method comprising: receiving adata frame, the data frame comprising: a legacy signal field including afirst symbol that is mapped onto a first binary phase shift keying(BPSK) constellation; a Very High Throughput Signal (VHT-SIG) fieldincluding a second symbol and a third symbol, the second symbol beingmapped onto the first BPSK constellation, and the third symbol beingmapped onto a second BPSK constellation, and the second BSPKconstellation being rotated by 90° counter-clockwise relative to thefirst BPSK constellation; and wherein the second symbol includes anindicator indicating Single User-Multiple Input Multiple Output(SU-MIMO) transmission or multi user-MIMO (MU-MIMO) transmission, andthe third symbol includes a cyclic redundancy check (CRC) for the secondsymbol and, at least a portion of, the third symbol.
 12. The method ofclaim 11, wherein the method is performed by a processor, and the dataframe is received by the processor from a wireless local area networktransceiver.
 13. The method of claim 11, wherein the third symbol isimmediately after the second symbol.
 14. The method of claim 11, whereinthe data frame is a Physical Layer Convergence Procedure Protocol DataUnit (PPDU).
 15. The method of claim 14, wherein the PPDU omits a highthroughput signal (HT SIG) field between the legacy field and theVHT-SIG field.
 16. The method of claim 11, wherein the VHT-SIG fieldfurther includes a multiplexing number indicating a number of spatialstreams.
 17. The method of claim 11, wherein the VHT-SIG field isimmediately after the legacy field.
 18. The method of claim 11, whereinthe second symbol is immediately after the first symbol.
 19. The methodof claim 11, wherein: the data frame is a PPDU; the PPDU omits an HT_SIGfield between the legacy field and the VHT-SIG field; the second symbolis immediately after the first symbol; and the third symbol isimmediately after the second symbol.
 20. An apparatus, comprising: atleast one wireless local area network transceiver; at least one antenna,communicatively coupled to the local area network transceiver; at leastone processor; at least one memory including computer program code, thememory and the computer program code configured to, working with theprocessor, cause the apparatus to perform at least the following:receive, by the local area network transceiver, a Physical LayerConvergence Procedure Protocol Data Unit (PPDU), the PPDU being receivedby the local area network by way of the antenna, the PPDU comprising: alegacy signal field including a first symbol that is mapped onto a firstbinary phase shift keying (BPSK) constellation; a Very High ThroughputSignal (VHT-SIG) field including a second symbol and a third symbol, thesecond symbol being mapped onto the first BPSK constellation, and thethird symbol being mapped onto a second BPSK constellation, and thesecond BSPK constellation being rotated by 90° counter-clockwiserelative to the first BPSK constellation; and wherein the second symbolincludes an indicator indicating Single User-Multiple Input MultipleOutput (SU-MIMO) transmission or multi user-MIMO (MU-MIMO) transmission;the third symbol includes a cyclic redundancy check (CRC) for the secondsymbol and, at least a portion of, the third symbol; the PPDU omits anHT_SIG field between the legacy field and the VHT-SIG field; the VHT-SIGfield further includes a multiplexing number indicating a number ofspatial streams; the second symbol includes the multiplexing numberindicating the number of spatial streams; the VHT-SIG field isimmediately after the legacy field; the second symbol is immediatelyafter the first symbol; and the third symbol is immediately after thesecond symbol.